The problems associated with the lubrication of automatic and manual transmissions and the operation of hydraulic fluid systems are well known to those skilled in the art. For example, in the lubrication of transmissions, proper fluid viscosity at both low and high temperatures is essential to successful operation. Good low temperature fluidity eases cold weather starting and insures that the hydraulic control system will properly "shift gears". High viscosity at elevated temperatures insures pumpability and the satisfactory functioning of converters, valves, clutches, gears and bearings.
In the operation of hydraulic fluid systems, proper fluid viscosity at both low and high temperatures is essential to successful operation. High temperature viscosity retention is beneficial in lubrication, contributes to streamline flow and reduces leakage. Good low temperature fluidity provides rapid control action, less heating loss and lower pressure drop.
These conflicting fluidity requirements call for a product that exhibits the following characteristics:
(A) high temperature viscosity retention,
(B) low temperature fluidity, and
(C) shear stability.
In order to prepare lubricants having these characteristics, it has become common practice to add a variety of chemicals to the oil. For example, in order to meet the viscosity requirements, compositions have been added to the oils which are characterized by relatively small change in their viscosity with changing temperature, and they are commonly graded according to SAE standards according to the viscosities at low (e.g., 0.degree. F) and at high temperatures (e.g., 210.degree. F). As a result of the incorporation of such additives, the lubricating oils are often referred to as being "multi-graded". In terms of widely accepted concepts, such multi-graded lubricants have the desirable properties as being able to function immediately, though cold, upon being put into service, and to continue to function satisfactorily as they become heated during operation.
It is desirable, if not necessary, that lubricating compositions especially designed for use as transmission fluids, hydraulic fluids, and gear fluids exhibit shear stability. Shear stability means that the lubricating oils will not degrade or lose their desirable viscosity characteristics as a result of the shearing forces encountered during their use. Lubricating oil compositions exhibiting desirable shear stability will be found to generally retain their viscosity within 85-100% of the original viscosity after a number of hours, (e.g., 100 hours) of service. It has been recognized that many ordinary viscosity index improvers commonly added to crankcase lubricating oils, such as high molecular weight, polyisobutene and polyacrylates, do not possess the desired shear stability for use in improving the viscosity characteristics of transmission fluids and hydraulic fluids.
U.S. Pat. No. 3,702,300 issued to Coleman relates to lubricating compositions containing a nitrogen-containing mixed ester of a carboxy-containing interpolymer.
U.S. Pat. No. 3,933,761 issued to Coleman relates to a nitrogen-containing mixed ester of a carboxy-containing interpolymer and lubricants containing the same.
U.S. Pat. No. 3,956,149 issued to Coleman relates to a lubricant or fuel composition containing a nitrogen-containing ester of a carboxy-containing interpolymer.
U.S. Pat. No. 3,959,159 issued to Coleman relates to lubricating compositions containing a nitrogen-containing mixed ester of a carboxy-containing interpolymer.
U.S. Pat. No. 4,284,414 issued to Bryant relates to a crude oil composition containing mixed alkyl esters of a carboxy-containing interpolymer.
U.S. Pat. No. 4,604,221 issued to Bryant et al relates to a nitrogen-containing ester of a carboxy-containing interpolymer and lubricants containing the same.
U.S. Pat. No. 3,329,658 issued to Fields relates to nitrogen-containing esters of carboxy-containing copolymers with dispersancy properties.
U.S. Pat. No. 3,449,250 issued to Fields relates to nitrogen-containing ester of carboxy-containing copolymers with dispersancy properties.
U.S. Pat. No. 4,180,637 issued to Evani et al relates to a process for preparing a low molecular weight carboxy-containing copolymer.
U.S. Pat. No. 4,200,720 issued to Evani et al relates to a process for preparing a low molecular weight carboxy-containing interpolymer.
U.S. Pat. No. 3,085,994 issued to Muskat relates to a carboxy-containing interpolymer.
U.S. Pat. No. 3,388,106 issued to Muskat relates to a process for making a carboxy-containing interpolymer.
U.S. Pat. No. 3,392,155 issued to Muskat relates to a polyoxy alkylene glycol ester of a carboxy-containing interpolymer.